Demetallization of residual oils with polyphosphoric acids

ABSTRACT

Crude oil residua containing normally incident organometallic impurities, e.g., vanadium and nickel porphyrins, are subjected to extraction with anhydrous liquid polyphosphoric acid at elevated temperatures, whereby the metals in the oil are extracted into the acid phase in the form of metal chelates of the linear polyphosphoric acids. In a cyclic process, the extracted metals are recovered as sulfides by precipitation with H2S, and the regenerated polyphosphoric acid is recycled.

United States Patent [72] Inventors George L. Tilley; 2,308,001 l/l943Fomey 208/256 D ld C, Y b th f Full t C lif, 1,935,162 1 l/ 1933 Morrell208/265 [211 App]. No. 887,991 2,131,879 10/1938 Ault et al.. 208/256 22il m, 24, 1969 2,865,838 12/1958 Mills 208/90 [45] patfamed 1971 PrimaryExaminer-Delbert E. Gantz [73] Asslgnee Union Oil Company of CaliforniaAm-smm Examiner j Nelson Los Angeles, Attorneys-Milton W. Lee, RichardC. Hartman, Lannas S.

Henderson, Dean Sandford and Robert E. Strauss [54] DEMETALLIZATION OFRESIDUAL OILS WITH POLYPHOSPHORIC APIDS ABSTRACT: Crude oil residuacontaining normally incident 12 3 Drawing organometallic impurities,e.g., vanadium and nickel 52 US. Cl 208/252, p p y are subjected oextraction with anhydrous liquid 203 90 203 25 polyphosphoric acid atelevated temperatures, whereby the [51] Int. Cl Cl0g 17/00 metals in theoil are extracted int he acid phase in the form 501 Field oiSearch208/252, of metal chelates of the linear P yp p acids- In a 256, 265,90, 98, 219 cyclic process, the extracted metals are recovered assulfides by precipitation with H 8, and the regenerated polyphosphor-[56] References Cited ic acid is recycled.

UNITED STATES PATENTS 2,682,496 6/1954 Richardson et a1. 208/90 L4 q aCRUDE 0/2 AIM/[0P #75 46705 rm/'1 fl T 2 D .v-g /&

6' 0 MITJA div/DES PATENTEDunv 23 197: 3.622.505

SHEET 1 OF 2 H2 0 F/? M574; v

OX/DES INVENTORS 650/765 4. 7744:? 004/440 6. you/v6 BY 'Z MM 5 ATTORNEYDEMETALLIZATION OF RESIDUAL OILS WITH POLYPI'IOSPIIORIC ACIDS BACKGROUNDAND SUMMARY OF THE INVENTION The primary refining of crude oils byfractional distillation (either at atmospheric or reduced pressures)always leaves behind an undistillable residue which has been a challengeto petroleum refiners for many years. It may represent from 5 to 50volume-percent of the original crude oil, and there is hence aconsiderable economic incentive in converting it to more valuabledistillate products. Thermal coking can be utilized to recover someadditional relatively low value distillate products, but the yield islow and a considerable portion of the residuum is converted to coke.Products such as asphalt, paraffin waxes and lube oils can be recoveredfrom such residua but the demand for these products is insufficient toconsume more than a minor proportion of the available residua. As aresult of all these factors, probably the major portion of petroleumresidua has in the past been diverted to low value products such as fueloils.

A possibility which has long intrigued petroleum refiners has been toeffect a substantially total conversion of petroleum residua to highvalue distillate products such as gasoline, turbine fuels, diesel fuelsand the like. To achieve this objective requires that the asphalticconstituents thereof, instead of being converted to coke as in thermalcoking, be first hydrogenated so that they can be cracked orhydrocracked without rapidly coking up the catalyst employed. A majordifficulty encountered in any initial hydrogenation of such residuaresides in the rapid fouling of the hydrogenation catalyst brought aboutby the metallic constituents in the residua. Metals commonly found incrude oils include vanadium, arsenic, nickel, copper and iron, and theyevidently occur mainly in the fonn of soluble chelates with variousorganic compounds such as porphyrins. Vanadium and arsenic areparticularly potent poisons for most of the conventionally usedhydrogenation catalysts. Much effort has been devoted in the past to thedevelopment of methods for demetallizing petroleum residua so as torender them amenable to catalytic hydrogenation, but for various reasonsnone of these methods have proven to be economically satisfactory.

In addition to the economic incentive of upgrading residua intodistillate products, air pollution problems have in recent years made itmore difficult to dispose of residua even as fuel oils, primarilybecause of the high sulfur content thereof. The most practical means ofremoving sulfur is by catalytic hydrodesulfurization, but here again theproblem of catalyst poisoning by the metallic constituents is a majordeterrent.

One of the most commonly suggested methods of removing metals from oilsis by extraction with strong acids. Acids which have been suggestedinclude the hydrogenhalides, sulfuric acid alkyl and aryl sulfonicacids, fluorophosphoric acid, and orthophosphoric acid (see U.S. Pat.No. 2,682,496). Although these acids can effect nearly completedemetallization, their use presents serious problems. At the hightemperatures required to reduce the viscosity of the undiluted residualoils sufficiently for convenient processing, the sulfonic acidsdecompose, and the hydrogen halides require high pressure vessels.Moreover, all of the above acids except orthophosphoric will, at thetemperatures required herein, chemically attack olefins, resulting inreduced yields and loss of acid. orthophosphoric acid also requires ahigh-pressure extraction vessel at the temperatures here required, inorder to prevent the evolution of water vapor (which would disrupt theefficiency of a countercurrent extraction column).

We have now discovered that all of the foregoing acid extractionproblems can be avoided by the use of a specific class of polyphosphoricacids wherein the mole ratio of Ibo/P is less than 3.0 but greater thanabout 1.2, which corresponds to a P 0 content of about 72.5 to 86.5weight-percent, on an impurity free basis. Acids of this character arefound to present several unique advantages. Firstly, as compared toorthophosphoric acid, they extract metals, particularly vanadium, muchmore efficiently and completely, and can be used at temperatures above500" F. and at atmospheric pressure without generating a vapor phasewhich would upset the contacting efficiency of a countercurrentextraction column. Secondly, as compared to the sulfonic acids, they arecompletely stable at the required temperatures. Thirdly, as compared toany of the above enumerated acids except orthophosphoric, they do notattack olefins. Finally, their specific gravity is such that they can bereadily separated from the demetallized oil.

The unique efficiency of the polyphosphoric acids as metal extractantsis believed to stem from their chelating properties. To form chelates itis necessary that a substantial proportion of the phosphorus in the acidbe present in the fonn of linear poly acids, e.g., pyrophosphoric acid,tri-, tetra-, and pentapolyphosphoric acids. The acid having an PLO/P 0mole ratio of 3.0 contains 72.5 weight-percent P 0 and about I 1.5percent of the total phosphorus is present as linear polyacids. The acidhaving an H O/P 0 mole ratio of 1.2 contains 86.5 weight-percent P 0 andabout 97 percent of the total phosphorus is present as linear polyacids.

BRIEF DESCRIPTION OF DRAWINGS FIG. 1 is a simplified flow diagramillustrating a specific adaptation of the invention utilizing continuouscountercurrent extraction with regeneration and recycle of the acid.

FIG. 2 is a graph illustrating the data hereinafter set forth in exampleII.

FIG. 3 is a graph depicting the data set forth hereinafter in exampleIII.

DETAILED DESCRIPTION A. Description of Acids The polyphosphoric acidsutilized herein can be derived from commercial wet process phosphoricacid, but preferably they are produced by concentrating furnace gradewhite phosphoric acid. The wet process acid normally contains about25-55 weight-percent of P 0 and is contaminated with metal salts andfluorine, while the white acid is substantially free of metallicimpurities and fluorine. In either case, the commercial acid issubjected to conventional concentration procedures involving essentiallythe boiling off of water until the desired P concentration is attained.Operative and preferred acids may be characterized as follows, on animpurity free basis:

The acid of minimum strength above, i.e., containing 72.5 weight-percentP 0 corresponds to nominally 100 percent H PO However, as is well known,this material actually contains a small amount of polymer acid as wellas free water. The presence of free water appears to be undesirable inour process, and hence acids of at least about 75 weight-percent P 0 arepreferred, since they contain substantially no free water. The acidscontaining from 83 to 86.5 percent P 0 are very efl'icient extractionagents, but they are not preferred because of their extremely highboiling points, ranging above 1,000 F. Since, as will be notedhereinafter, it is preferred to add water to the extract acid andrafiinate oil phases in order to promote better phase resolution, itbecomes necessary in a cyclic process to evaporate off the excess waterto obtain a concentrated acid for recycle. This becomes a difficult andexpensive operation when it is desired to maintain acids of greater thanabout 83 percent P 0 in the extraction zone. But

from the standpoint of simple operability, the acids containing up to86.5 percent or more of P,O are very effective. The

boiling points of several of the preferred acids for use herein are asfollows:

Table 2 Wt.-% Boiling I50 Point, 'F.

B. Feedstocks The residual oils employed herein as feedstocks comprisethe bottoms fraction from conventional fractional distillation or vacuumdistillation of substantially any petroleum crude oil. They are dark,viscous materials rich as asphaltenes, sulfur and nitrogen compounds;their pour point may range between about 50 and 300 F. The solventextraction of such oils has in the past presented serious handlingdificulties due to the viscosity problem. In the past, when strong acidssuch as sulfuric were utilized (which chemically attack the oils at hightemperatures), it was necessary to employ a light hydrocarbon diluent inorder to reduce viscosity sufficiently for low temperature operations.The use of a diluent is undesirable because of the required increase inequipment capacity and the increased expense involved in the heating,cooling and product recovery operations. One of the unique advantages of35 the polyphosphoric acids is that they can be employed at sufiicientlyhigh temperatures to reduce the viscosity of the oil to a level suchthat inert diluents are not required. However the optional use of adiluent is not excluded.

in other respects, the residual oil feedstocks employed herein may becharacterized as comprising at least about50 volume percent ofcomponents boiling above about l,000 F., and having an API gravity belowabout C. Process Conditions The extraction may be carried out employingconventional contacting procedures generally used in solventextractions; either -single stage batch operations with suitableagitation to obtain intimate contacting of the two phases, ormultistage, continuous countercurrent extractions may be utilized. Thelatter form of contacting is generally preferred, and may consist inpassing the hot oil upwardly through a column packed with glass beads,Raschig rings or the like, countercurrently to a descending stream ofpolyphosphoric acid. Although it is possible to remove more than 90percent of the metal content of the oil in a single stage of contacting,it is generally preferable from the standpoint of reducing contact timeand increasing metals removal to substantially 100 percent, to employ atleast two, and preferably three to about 10 theoretical contactingstages. The principal operative and preferred conditions for thecontacting are summarized as follows:

TABLE 3 Broad Range Preferred Range Temp.. F. 350-750 450-700 Pressure.p.s.i.g. 0-2,000 0-50 Contact Time, Minutes 2-l00 10-45 Solvent/OilRatio, V/V 0.0l-0.5 0.024).!

Those skilled in the art will understand that the more extended contacttimes-in the above ranges will be utilized in conjunction withrelatively low temperatures and/or inefficient contacting techniques,while the shorter contact times will ordinarily be utilized inconjunction with higher temperatures and eflicient contactingprocedures. Also, for an equivalent degree of metals removal, longercontact times will be required for a single stage operation than formultistage operations. it is preferred to adjust the extractionconditions so as to remove at least about 99 percent of the vanadiumcontent of the oil, and at least about 75 percent of the nickel content.

During the contacting, particularly if substantial agitation occurs,there will be some emulsification of oil in the acid phase, and of acidin the oil phase, thus creating a separation problem. One effectivemethod for improving phase resolution consists in adding sufficientammonia to the aqueous phase to raise the pH to about 6-8, whereby aclear, colorless aqueous phase is obtained. However, the resultingammonium phosphate solution must then either be diverted to other uses,or subjected to high temperature regeneration procedures for recoveringthe ammonia and acid. A more desirable method consists simply in addingwater, either to the two-phase system, or to each phase separately, withsuitable agitation as could occur for example in a transfer line,followed by a simple gravity separation. Large amounts of water are notrequired; generally we add only sufficient water to provide an aqueousphase containing about 20-50 weight-percent P,O,, preferably about 25-35weight-percent.

When water is added to the system to promote phase separation, hightemperatures, above about 400 F. must still be maintained in order tokeep the oil phase in a sufficiently nonviscous state. This entails apressure separation system from which the aqueous acid phase may berecovered, treated for metals recovery by any desired method (one ofwhich will be described hereinafter), and then flashed into a lowpressure heating zone to evaporate off water and recover thepolyphosphoric acid for recycle. In some cases it may be desirable tosubject the raffinate oil phase to a final water wash in order to removetraces of remaining acid.

D. Description of FIG. 1

FIG. 1 illustrates a specific adaptation of the process embodyingcontinuous countercurrent contacting, and recovery of the extractedmetals from the acid extract by precipitation with hydrogen sulfide. Theinitial crude oil is brought in via line 2 and fractionated indistillation column 4, from which distillate products are withdrawn vialine 6. The remaining residual oil is withdrawn as bottoms via line 8and passed in part through flash evaporator 10 in heat exchangerelationship to recycle acid as will be described hereinafter, and inpart via bypass line 12 and line 14 to the bottom of countercurrentextraction column 16, in which it flows upwardly, countercurrently todescending recycle and makeup polyphosphoric acid admitted to the top ofthe column via lines 18 and 20. Contacting in column 16 occurs under theconditions above described, preferably at atmospheric pressure.

From extraction column 16, the acid extract is withdrawn via line 22 andpressured via pump 24 into water line 26 containing makeup and recyclewater from lines 28 and 30. The temperature of the water in line 28 ispreferably adjusted so that upon mixing with the acid extract in line26, the resulting temperature will be in the range of about 400 to 600F., preferably 400 to 500 F whereby the autogeneous pressure in line 26to maintain the water in liquid phase will range between about 250 and650 p.s.i.g. The dilute acid in line 26 is then mixed with thedemetallized rafiinate oil which is withdrawn from extraction column 16via line 32, precooled to the desired temperature in heat exchanger 34and pressured via pump 36 into line 26. The combined mixture of acidextract and raffinate oil in line 26 is then transferred to a separatingvessel 40, from which the upper phase of demetallized oil is withdrawnvia line 42.

The lower phase of acid extract in separator 40 is withdrawn via line 44and mixed therein with recycle and makeup hydrogen sulfide brought invia lines 46 and 48. Sufficient hydrogen sulfide is added at this pointto saturate the aqueous acid, thereby effecting maximum precipitation ofthe metals as sulfides. Due to the high acidity of the extract in line44, complete metals precipitation ordinarily cannot be obtained, butthis is not a critical factor; it is necessary to precipitate onlyenough of the metals as sulfides to provide an acid for recycle whichhas sufiicient capacity to economically chelate and extract additionalmetals from the oil. If the metals concentration in the extract in line44 is high, satisfactory precipitation of e.g., 50 to 90 percent of themetals can be obtained by merely saturating the acid with H 8. However,if the metals concentration in the extract is low, it may be desirableto raise the pH of the acid stream somewhat in order to drive the metalsulfide precipitation further toward completion. For this purpose,sufficient ammonia may be introduced with the hydrogen sulfide in line48 to raise the pH of the acid extract in line 44 to e.g., about 4 to 8.

The resulting slurry of precipitated metal sulfides and aqueous acid inline 44 is then passed through a suitable filter 50 (or other suitableseparating means such as a centrifuge) from which the solid metalsulfides are withdrawn via line 52 and sent to a suitable roaster 54which converts the sulfides to metal oxides withdrawn via line 58.Sulfur dioxide taken off via line 56 may be sent to suitable sulfurrecovery facilities not shown. The demetallized acid from filter 50 isthen passed via line 60 and pressure-reducing valve 62 into flashseparator 64, from which excess H 8 is withdrawn via line 46 andrecycled as previously described. If no ammonia was added to the systemvia line 48, a substantially complete flashoff of H S is obtained invessel 64 by simply reducing the pressure by about 50-100 p.s.i.g.,giving a substantial steam stripping effect.

The stripped acid in separator 64 is then transferred via line 66 andpressure reducing valve 68 to flash evaporator in which water vapor andany remaining H 8 and ammonia are flashed off at substantiallyatmospheric pressure and withdrawn via line 70. Evaporator 10 thus actsas a concentrator to remove the water which was added to the system vialine 28 and return the acid to the desired strength for use inextraction column 16. The degree of concentration in evaporator 10 iscontrolled automatically by means of motor valve 72 in bypass line 12which is operated by temperature recorder controller 74 in response totemperature detected in line 18. Temperature recorder controller 74 iscalibrated so as to maintain a temperature in line 18 corresponding tothe atmospheric boiling point of the desired concentration of acid byopening or closing valve 72, thus forcing a smaller or larger proportionof hot residual oil through flash evaporator 10.

The steam in line 70 may, if it is free of hydrogen sulfide and ammonia,be recycled directly to line 28, but in cases where ammonia was addedvia line 48, both ammonia and H 8 will be dissociated and driven offfrom the acid being concentrated in evaporator 16. To recover suchammonia and H S, the steam in line 70 may be passed through a cooler 72to condense out, e.g., about 5 to 10 percent of the water content andthereby absorb the bulk of the H 8 and ammonia. The resulting steam andaqueous ammonium sulfide condensate is passed into separating vessel 74,from which the condensate is withdrawn via line 76 and recycled via line48. The remaining steam is taken off via line 78, treated for heatrecovery and condensation in heat exchanger 80 and then either exhaustedas waste water via line 82 or recycled to line 28 via line 30 aspreviously described.

EXAMPLE I As a specific illustration of our process carried out withcountercurrent contacting as described in connection with FIG. 1, a 17percent vacuum residuum feedstock derived from a California crude oil isutilized, having the following principal characteristics:

Gravity, APl 6.5 Pour Point, F. 200 Sulfur, wt-% 2.37

Nitrogen, wt-% LI Fe, ppm. 97 Ni, p.p.m. I50 V, p.p.m. I

To achieve 99+ percent removal of vanadium and 90+ percent removal ofnickel from this oil, while achieving at least percent recovery ofdemetallized oil, the following operative conditions are found to besuitable:

Wt-% P,O in Acid 76 Number of Extraction Stages 3 Acid/Oil Ratio, V/Vl/l0 Extraction Temp., F. 650 Contact Time in Extraction Column. MinutesEXAMPLE ll The vacuum residuum feed described in example 1 was subjectedto several single stage batch extractions with polyphosphoric acids ofvarious P 0, concentrations. In each case a constant 6.6/1 weight-ratioof oil/P 0 was maintained, with the acid and oil being placed in aquartz lined bomb under nitrogen and rocked at 650 F. for 2 hours. Aftercooling, sufificient water was added to give an acid phase containing 30weight-percent P 0 and the contents were then heated to 500 F. androcked for 1 hour, allowed to settle for 1 hour and then cooled.Analyses of the resulting oil phases gave the following data:

The above data, which is plotted in FIG. 2, shows that the efficiency ofextraction of vanadium increases substantially linearly with the degreeof polymerization (and hence chelating capacity) of the acid. Nickel, onthe other hand, appears to be present in two different forms: one whichis easily extracted by nonchelating acids (Runs 1-3), and another whichrequires a chelating acid (Runs 4,5

It should be noted that the foregoing runs do not represent optimumcommercial extraction conditions. They were designed merely to assurethe attainment of equilibrium in each case, so that the relativeefficiency of the acids could be compared.

EXAMPLE III A 35 percent atmospheric residuum derived from a Californiacrude oil and having a pour point of 85 F., and an AP] gravity of 10.7"was subjected to a series of extractions similar to those of example llexcept that the contact time in each case was 4-5 hours and the bombtemperature was 450 F. The demetallized oils were recovered bycentrifugation instead of water dilution and settling as in example II.Analysis of the resulting oils gave the following data:

The foregoing data is plotted in FIG. 3, and shows in general the sameincreased extraction efficiency with increasing acid polymerization aswas shown in example ll.

EXAMPLE lV TABLE 6 Percent demetal- Oil/acld lization Run ratio, No.Acid used v./v. Ni V 10 Polyphosphorlc (76% P105) 2/1 70 76 11- Methanesullonlc 2/1 22 21 13-. Polyphosphorlc (76% P205)--. 10/1 61 72 14Methane sulfonic 10/1 20 23 It is clearly evident that thepolyphosphoric acids are much moreefficient demetallizing agents than ismethane sulfonic acid.

Additional modifications and improvements utilizing the discoveries ofthe present invention can readily be anticipated by those skilled in theart from the foregoing disclosure, and such modifications andimprovements are intended to be included within the scope and purview ofthe invention as defined in the following claims.

We claim:

1. A process for demetallizing a crude oil residuum contaminated with atleast one of the metals, nickel, vanadium and iron or compounds thereof,which comprises intimately contacting said residuum with an extractionsolvent consisting essentially of liquid polyphosphoric acidconcentrated to a P,O equivalent above about 72.5 but below about 86.5weight-percent, said contacting being carried out at a temperaturebetween about 350 and 750 F. to thereby effect a substantial extractionof metals from said residuum, and recovering from said contacting asubstantially demetallized 8 product oil.

2. A process as defined in claim 1 wherein said polyphosphoric acidcontains a P 0 equivalent between about 75 and 83 weight-percent.

3. A process as defined in claim 1 wherein said extraction is carriedout at a temperature between about 450 and 700 F.

4. A process as defined in claim 1 wherein said contacting is carriedout in the absence of a hydrocarbon diluent for said residuum.

5. A process as defined in claim 1 wherein said polyphosphoric acidcontains a P 0 equivalent between about 75 and 83 weight-percent, andwherein said contacting is carried out at a temperature between about450 and 700 F. in the absence of a hydrocarbon diluent for saidresiduum.

6. A cyclic process for demetallizing a crude oil residuum contaminatedwith at least one compound of a metal from the class consisting ofvanadium and nickel which comprises:

1. intimately contacting said residuum with an extraction solventconsisting essentially of a liquid polyphosphoric acid concentrated to aP,O, equivalent above about 72.5 but below about 86.5 weight-percent,said contacting being effected at a temperature between about 350 and750 F;

2. separating from said contacting an acid extract phase containingextracted metals and a substantially demetallized raffinate oil phase;

. adding sufficient water to said acid extract phase to reduce the P 0,concentration thereof to between about 20 and 50 weighbpercent, therebyreleasing additional emulsified oil from said acid extract and forming asecondary raffinate phase and a substantially hydrocarbon-free aqueousacid phase;

4. separating said secondary raffinate from said aqueous acid phase;

5. removing at least a substantial portion of the metal content fromsaid aqueous acid phase;

6. evaporating sufficient water from the aqueous acid phase from step(5) to concentrate the same to a P 0, content above about 72.5 but belowabout 86.5 weight-percent; and (7) recycling the concentrated acid fromstep (6) to step l 7. A process as defined in claim 6 wherein the P 0concentration of said polyphosphoric acid recited in steps l and (6) isbetween about 75 percent and 83 percent.

8. A process as defined in claim 6 wherein said extraction step (1) iscarried out at a temperature between about 450 and 700 F.

9. A process as defined in claim 6 wherein said extraction step l) iscarried out in the absence of a hydrocarbon diluent for said residuum.

10. A process as defined in claim 6 wherein said contacting step l) iscarried out in continuous countercurrent fashion.

11. A process as defined in claim 6 wherein said removal of metals instep (5) is effected by the addition thereto of hydrogen sulfide toprecipitate the metals as sulfides.

12. A process as defined in claim 6 wherein said contacting step l) iscarried out in continuous countercurrent fashion in the absence of ahydrocarbon diluent for said residuum, at a temperature between about450 and 700 F., and wherein said polyphosphoric acid has a P 0equivalent between about 75 and 83 weight-percent.

2. separating from said contacting an acid extract phase containingextracted metals and a substantially demetallized raffinate oil phase;2. A process as defined in claim 1 wherein said polyphosphoric acidcontains a P2O5 equivalent between about 75 and 83 weight-percent.
 3. Aprocess as defined in claim 1 wherein saId extraction is carried out ata temperature between about 450* and 700* F.
 3. adding sufficient waterto said acid extract phase to reduce the P2O5 concentration thereof tobetween about 20 and 50 weight-percent, thereby releasing additionalemulsified oil from said acid extract and forming a secondary raffinatephase and a substantially hydrocarbon-free aqueous acid phase; 4.separating said secondary raffinate from said aqueous acid phase;
 4. Aprocess as defined in claim 1 wherein said contacting is carried out inthe absence of a hydrocarbon diluent for said residuum.
 5. A process asdefined in claim 1 wherein said polyphosphoric acid contains a P2O5equivalent between about 75 and 83 weight-percent, and wherein saidcontacting is carried out at a temperature between about 450* and 700*F. in the absence of a hydrocarbon diluent for said residuum. 5.removing at least a substantial portion of the metal content from saidaqueous acid phase;
 6. evaporating sufficient water from the aqueousacid phase from step (5) to concentrate the same to a P2O5 content aboveabout 72.5 but below about 86.5 weight-percent; and (7reconcentratedrecycling the concentrated acid from step (6) to step (1).
 6. A cyclicprocess for demetallizing a crude oil residuum contaminated with atleast one compound of a metal from the class consisting of vanadium andnickel which comprises:
 7. A process as defined in claim 6 wherein theP2O5 concentration of said polyphosphoric acid recited in steps (1) and(6) is between about 75 and 83 percent.
 8. A process as defined in claim6 wherein said extraction step (1) is carried out at a temperaturebetween about 450* and 700* F.
 9. A process as defined in claim 6wherein said extraction step (1) is carried out in the absence of ahydrocarbon diluent for said residuum.
 10. A process as defined in claim6 wherein said contacting step (1) is carried out in continuouscountercurrent fashion.
 11. A process as defined in claim 6 wherein saidremoval of metals in step (5) is effected by the addition thereto ofhydrogen sulfide to precipitate the metals as sulfides.
 12. A process asdefined in claim 6 wherein said contacting step (1) is carried out incontinuous countercurrent fashion in the absence of a hydrocarbondiluent for said residuum, at a temperature between about 450* and 700*F., and wherein said polyphosphoric acid has a P2O5 equivalent betweenabout 75 and 83 weight-percent.